1. Technical Field
The present invention relates to a drive unit for a reverse-conducting switching element, the drive unit including a chip on which a voltage-controlled switching element is connected in anti-parallel to a free-wheel diode.
2. Related Art
In related arts, reverse-conducting insulated gate bipolar transistors (RC-IGBTs) incorporating a diode are well known. Such an RC-IGBT includes a chip on which an IGBT is connected in anti-parallel to a free-wheel diode. It is well known that, in an RC-IGBT, voltage drop in the free-wheel diode when forward current passes therethrough increases with the application of a voltage to the gate of the RC-IGBT. The increase of voltage drop accelerates power loss in the free-wheel diode when forward current passes therethrough. The acceleration of the power loss may lead to the increase of heat generated by the RC-IGBT. Resultantly, the reliability of the RC-IGBT may be impaired.
A technique as a measure against this problem is also well known as disclosed in a patent document JP-A-2009-268054. In this technique, a drive signal inputted to the gate of an RC-IGBT is prevented from passing therethrough in a period in which current is determined to pass through a free-while diode in a forward direction to thereby turn off the RC-IGBT (this control is hereinafter referred to as current feedback control). The current feedback control is able to prevent the increase of voltage drop in the free-wheel diode and, further, prevent the acceleration of power loss in the free-wheel diode.
Another technique related to driving of an. IGBT is also well known as disclosed in a patent document JP-B-3339311. In this technique, the rate of charge/discharge of gate charge with respect to an IGBT is changed sometime in a period from when the charge/discharge of gate charge is started until when it is completed (this control is hereinafter referred to as active gate control (AGC)). The following is a specific description of this technique, taking as an example a charge process of a switching element. Specifically, a switching element has a gate to which a pair of charging paths is connected. The two charging paths are provided with respective resistors having a resistance different from each other. Each of the charging paths has a transistor that opens/closes the charging path.
With this configuration, a turn-on command is inputted to the switching element to turn on the transistor in a path that includes the higher-resistance resistor. Thus, gate charges are charged at a low charge rate. After that, the above transistor is turned off, followed by turning on the other transistor in the other path that includes the lower-resistance resistor to increase (change) the charge rate. Thus, gate charges are charged at a high charge rate.
According to the techniques set forth above, the increase of surge voltage is suppressed, which is caused in bringing the switching element into an on state from an off state, or vice versa. In addition, the techniques can contribute to reducing switching loss.
Here, an idea of applying the active gate control to an RC-IGBT is considered. In a drive unit of an RC-IGBT, which can perform the active gate control, a transistor used for charging gate charges may be erroneously turned on for some reason, for example, to erroneously perform the active gate control. When the active gate control is erroneously performed, the RC-IGBT may be erroneously turned on. If the period in which the RC-IGBT is erroneously turned on coincides with the period of performing the current feedback control, power loss in the free-wheel diode is accelerated, which may lead to the increase of the amount of heat generated by the RC-IGBT.